This invention relates generally to coil forms, and more particularly to coil forms having winding surfaces of high dimensional accuracy, and a method and apparatus for the manufacture thereof.
In manufacturing certain types of electronic equipment such as radio frequency transmitters and receivers the need exists for inductance coils which can be economically manufactured in large quantities with consistent and predictable electrical characteristics. This is particularly true with coils which are self-resonant at RF frequencies, such as those used in antenna tuning applications, wherein the distributed capacitance of the coil is relied on to form a resonant circuit and no variable tuning means are provided.
While injection molding techniques allow forms for such coils to be produced at greatly reduced cost, the technique has been generally successful only for coils having a very small diameter; i.e. approximately 0.5 inch or less. For coil forms of larger sizes, e.g., from approximately 0.5 to 3 or more inches in diameter, the uneven shrinkage of the plastic following the molding operation has prevented the formation of a surface capable of supporting and positioning windings with the necessary precision to insure a consistent and predictable self-resonant frequency. The problem is compounded in applications such as the aforementioned antenna tuning application wherein the coil form must, in addition to being dimensionally accurate, be capable of withstanding extreme temperature variations and repeated mechanical stresses.
In his copending application, Ser. No. 639,421, filed Dec. 10, 1975, applicant describes a coil form which overcomes the above drawbacks by providing a cylindrical core member molded of a solid mass of plastic or similar material, and a thin plastic over-layer molded over the core for supporting the windings of the coil assembly. Since only the thin over-layer is subject to shrinkage during formation of the winding support surface, and not the greater core diameter, the net dimensional variation in the winding support surface of the completed coil form is very small.
The present invention is directed to improvements in this coil form, and to a method and apparatus for economically manufacturing the coil form for high volume applications.
It is therefore an object of the present invention to provide a new and improved injection-molded coil form having a precision winding support surface.
It is another object of the present invention to provide a new and improved injection-molded coil form having integral means for establishing electrical connection to an inductance coil wound thereon.
It is another object of the present invention to provide a new and improved method for economically manufacturing the coil form of the invention.
It is another object of the present invention to provide a new and improved method for manufacturing the coil form of the invention with improved dimensional accuracy.
It is another object of the present invention to provide a new and improved apparatus for manufacturing a coil form in accordance with the invention.